Stable isotope fractionation in live benthic foraminifera from the southern California Borderland

Abstract

Live specimens of benthic foraminifera have been analyzed from surface sediment samples collected from 108 to 1134 m depth in the Southern California Borderland. The temperature and isotopic composition of the ambient water and dissolved inorganic carbon have also been measured to permit comparison of foraminiferal compositions to expected equilibrium values. The species analyzed break down into four groups which reflect the influence of mineralogy (aragonite vs. calcite), biological fractionation (“vital effect”), and microhabitat effects on isotopic composition. The aragonitic Hoeglundina elegans show an enrichment in 18O relative to equilibrium calcite of 0.41%0 in shallower water increasing to at least 1.06%0 in deeper water. The 13C enrichment of H. elegans relative to the dissolved inorganic carbon (DIC) is also depth dependent, increasing from 1.27%0 at 108 m to 2.13%0 at 940 m. This depth dependence results from the change in composition of the DIC; the 13C composition of H. elegans remains constant with depth for some unknown reason. One possible explanation is that the change in composition of the DIC is balanced by an increase in Hoeglundina—HCO−3 fractionation with decreased temperature. The δ13C values of the Cassidulina species are almost identical to those of the ambient dissolved inorganic carbon (DIC) and reflect the decrease in the δ13C of the DIC with depth. The δ13C values of species not in 18O equilibrium vary independently of the changing composition of the DIC. There is a correlation between δ13C and δ18O among coeval species, consistent with the model which explains vital effect as the result of incorporation of isotopically light metabolic CO2 into the shell. The magnitude of disequilibrium is relatively constant for 18O but not for 13C, suggesting that in isotopic stratigraphy studies, it may be difficult to “calibrate” the δ13C of different species as has been done with δ18O. There appears to be a relationship between taxonomic group and vital effect. The foraminifera in approximate 18O equilibrium with the water, including species of Cassidulina, Bolivina, Uvigerina, and Globobulimina, are members of the family Cassidulinidae or superfamily Buliminacea. Those not in 18O equilibrium, including species of Pyrgo, Quinqueloculina, Triloculina, and Lenticulina, belong to the families Miliolidae and Vaginulinidae. No disequilibrium species have been found as yet in the low oxygen environments of the borderland. This observation and published findings on the ultrastructure of benthic foraminifera suggest that isotopic “behavior” may be related to a foraminifer's ability to exchange gases rapidly with its ambient water.